1. Field of the Invention
This invention relates to a projection type liquid crystal display device for full color representation, and more particularly to a structure of a liquid crystal panel used in the display device.
2. Description of the Related Art
As the liquid crystal display panel capable of displaying a fully colored picture, there is brought into practical use a liquid crystal panel having a multiplicity of picture (image) cells are disposed therein in a matrix, which liquid crystal panel serves as light shutters and in which liquid crystal panel tri-color filters of RGB (red-green-blue) are disposed for each picture cell. For keeping the display of the fully colored TV picture at a level comparable to the CRT (cathode ray tube), it is advantageous to use an active matrix type liquid crystal display panel, in which a switching thin film transistor and/or a diode are disposed in each picture element, in view of the requirement that the on-off control of the light shutters should be made at a 100% duty ratio by means of picture signal lines and scanning lines. The manufacture of these active matrix type liquid crystal display panels, however, relies on the so-called semiconductor process including film formation and photofabrication. Therefore, it is generally difficult to make a large size panel. In contrast to this, such scale-up can be easily achieved in the projection type liquid crystal display device in which liquid crystal panels are used as light valves to project an image on a screen.
In the projection type liquid crystal display device, there are one type in which a single sheet of the liquid crystal panel provided with RGB tri-color filters for each picture cell is used to project an image on a screen, and another type in which tri-color light beams of RGB are transmitted through three TN liquid crystal panels to project the images on a screen.
When tri-color light beams of RGB are transmitted through three TN type liquid crystal panels and the images produced by the TN type liquid crystal panels are projected on a single same screen, it is necessary to use TN type liquid crystal panels which are independently optimally designed from the optical viewpoint for each of RGB colors, since the polarization of the emanating light beams varies depending on the wavelength of the incident light beam to the TN type liquid crystal panel. The transmission characteristics of the normally black TN cells are theoretically described by Gooch and Tarry in J. Phys., D8, 1575 (1975). In the case of 90.degree. TN cell, the transmission (which is used to signify "transmittance" here) T can be expressed as EQU T=sin.sup.2 1/2-(1+u.sup.2).sup.1/2 /(1+u.sup.2).
Here, parameter u can be defined as EQU u=2.DELTA.n.multidot.d/.lambda.
where .DELTA.n is the refractive index anisotropy of the liquid crystal, d is the gap length of the liquid crystal layer, and .lambda. is the wavelength. The minimum value of the transmission T varies depending on the wavelength. When the respective center wavelengths of RGB are set at 610 nm, 545 nm, and 450 nm and .DELTA.n =0.098, the calculated values of the transmission T with respect to the gap length (d) are shown in FIG. 2(b). In FIG. 2(b), curve 1 represents the transmission when the R light is transmitted. The transmission takes the minimum value at a gap length of 5.4 .mu.m. Similarly, curves 2 and 3 represent transmissions of the G light and the B light, which take the minimum values at a gap length 4.8 .mu.m and a gap length 4.0 .mu.m, respectively. Therefore, when the same liquid crystal having the same .DELTA.n value is used for RGB liquid crystal panels, the transmission characteristics can be optimized by adjusting the gap lengths of the respective liquid crystal panels to the above-mentioned values. It can also be considered that different liquid crystals of different .DELTA.n value may be used for RGB panels to optimize the transmission characteristics. Optimization of the transmission characteristics, while satisfying various practical properties required for the liquid crystal material, for example, the viscosity influencing the response, the anisotropy in the dielectric constant influencing the voltage vs. transmission characteristics, the elasticity, etc., may lead to a complicated system.
A display system in which tri-color lights of RGB are transmitted through TN type liquid crystal panels and projected on a screen was reported by S. Aruga in SID Int'l Symposium Digest Tech. Papers No. 18, p. 75 (1987), where the above-mentioned .DELTA.n.multidot.d/.lambda. is optimized for the respective panels.
According to the prior art method of optimizing the transmission characteristics by varying the gap lengths of the respective RGB liquid crystal panels, liquid crystal display panels having gap lengths of high precision of submicron order are necessary to realize a picture display device having a high contrast and a good color reproducibility. To make liquid crystal panels of different gap lengths for RGB lights beams is disadvantageous from the manufacturing technique and the production control. For example, the gap length or the thickness of a liquid crystal layer of a liquid crystal panel is maintained by spacers of glass fibers or beads. The gap length is dependent on the diameter, the material, and the distribution density of these spacer members, and further relates to the warp of the substrate glass constituting the liquid crystal panel, a pressing force applied to reform the warp of the substrate glass and to seal the liquid crystal panel, temperature condition, etc. Therefore, it is not easy to realize the required submicron precision gap lengths. Further, when the gap lengths of the TN type liquid crystal panels differ from one another, the response characteristics and the voltage-transmission characteristics as well as the optical characteristics will also differ disadvantageously from one another.